RESUMO
Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni_{55}Fe_{17}Ga_{28} alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2_{1} and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V_{Ni} is identified as the most probable vacancy present in Ni_{55}Fe_{17}Ga_{28}. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.
RESUMO
Magnetite nanoparticles have been synthesized at different temperatures in order to get nanoparticles of different average sizes. Powders of the synthesized nanoparticles were introduced in a radio frequency electromagnetic apparatus built to perform hyperthermia measurements in laboratory animals. The nanoparticles synthesized at 80 degrees C, the ones giving the largest specific absorption rate values, have been functionalized with different organic ligands to study the influence of functionalization on specific absorption rate values. In all the synthesized nanoparticles, with and without organic surroundings, specific absorption rate measurements have been performed to study the influence of applied magnetic field intensity an frequency.
RESUMO
Theoretical positron lifetime values have been calculated systematically for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, as well as different parametrizations for the positron enhancement factor and correlation energy. The results obtained have been studied and compared with experimental data, confirming the theoretical trends. As is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The effects of enhancement factors used in calculations have been commented upon. Finally, we have analysed the effects that f and d electrons have on positron lifetimes.
